How can I tell if someone’s hacked my Gmail account?

I’m getting paranoid that someone else is logging in to my Gmail account. Is there some way I can tell, or even just tell if there’s more than one logged in browser?

There seem to be two types of successful Gmail hack outcomes: either they immediately hijack your entire account (typically by changing your recovery email then password) or they tap into your resources by try to stay below the radar so that you continue to use the account too. The former case is easy to identify because, well, suddenly you can’t use your account. Ugh!

In the latter case, however, it can be tricky to figure out what’s going on and if they’re really careful, you won’t even see that messages you haven’t read are marked as read or similar telltale signs. The main way to tell that this has occurred is to keep a close eye on the very bottom of your Gmail home screen.